Ch. 4 Electricity, Magnetism, and electromagnetism Flashcards Preview

Bushong Radiology Physics > Ch. 4 Electricity, Magnetism, and electromagnetism > Flashcards

Flashcards in Ch. 4 Electricity, Magnetism, and electromagnetism Deck (141):
1

What is the primary function of the xray machine?

to convert electric energy into electromagnetic energy.

2

How is electric energy supplied to the xray machine?

In the form of well controlled electric current.

3

Where does the conversion of electric energy to heat and xrays take place?

in the xray tube

4

Electric charge comes in units that are...

positive or negative

5

What are the smallest units of electric charge?

protons and electrons

6

What is the charge of an electron?

one unit of negative charge

7

What is the charge of a proton?

one unit of positive charge

8

T or F? electrons are often free to travel from the outermost shell of one atom to another atom?

T

9

T or F? protons are fixed inside the nucleus of an atom and are not free to move?

T

10

Electrostatics is the study of...

electric charges

11

Why do we get shocked when we rub our feet across a carpet floor and then touch a doorknob?

Because electrons are rubbed off the carpet and onto your shoes, causing you to become electrified.

12

When is an object considered to be electrified?

When it has too many or too few electrons.

13

Matter has ____ and ___ ____. It may also have ____ ____.

mass, energy; electric charge

14

How can electrification be created? (3 ways)

contact, friction or induction

15

Where could some electrons be bound loosely, to be removed easily and cause electricity?

in the outer shell of an atom

16

What is an example of an electric ground and why?

the Earth is an example because it behaves like a huge reservoir for stray electric charges.

17

What is induction?

The process of making ferromagnetic material magnetic.

18

What is the smallest unit of electric charge?

the electron

19

What is the smallest fundamental unit of electric charge?

the coulomb (C) 1 C = 6.3 x 10^18

20

One ampere is the flow of...

one coulomb per second

21

Unlike charge ___ and like charges ___

attract; repel

22

Associated with each electric charge is...

an electric field

23

Do uncharged particles have a charge?

no

24

Describe electrostatic force

When the force of attraction between unlike charges or repulsion between like charges is attributable to the electric field

25

What is the following law? The electrostatic force is directly proportional to the product of the electrostatic charges and inversely proportional to the square of the distance between them.

Coulomb's law

26

Electric charge distribution is uniform....

throughout or on the surface

27

Electric charge of a conductor is concentrated where?

along the sharpest curvature of the surface

28

What kind of energy do electric charges have?

potential energy

29

What is the unit of electric potential?

Volt (V)

30

The higher the ___, the greater the ___ to do work

voltage; potential

31

What is electric potential in homes and offices?

110 V

32

What do xray machines require for electric potential?

220 V or higher

33

1 V = ?

1 J/C

34

What is it called when electric potential (V) is applied to an object and then electrons move along the object?

electric current or electricity

35

What is the study of electric charges in motion?

electrodynamics

36

the direction of electric current is always ____ that of electric flow.

opposite

37

What is a conductor?

any substance through which electrons flow easily

38

What is an insulator?

any material that does not allow electron flow

39

What led to the development of microchips and computer technology?

The discovery of the semiconductor materials like silicon (Si) and germanium (Ge)

40

What is a semiconductor?

a material that under some conditions behaves as an insulator and in other conditions behaves as a conductor.

41

At room temperature all materials...

resist the flow of electricity

42

When does the resistance of electric flow decrease?

when the temperature of material reduces

43

Define duperconductivity

the property of some materials to exhibit no resistance below a critical temperature

44

What is a superconducting material?

niobium and titanium

45

Superconductors do not hold true to what law?

Ohm's law

46

A superconducting circuit can be view as..

one in perpetual motion because electric current exists without voltage.

47

Why is energy needed for a superconductor if an electric current can exist without voltage?

Because for the material to behave as a superconductor it must be made very cold which requires energy.

48

What happens to a conducting wire if the diameter is reduced or if different material is inserted?

The electric resistance is increased

49

What constitutes an electric circuit?

When resistance is controlled and the conductor is made into a closed path

50

Increasing electric resistance results in a...

reduced electric current

51

What is electric current measure in?

amperes (A)

52

The ampere is proportional to the number of...

electrons flowing in the electric circuit

53

One ampere is equal to an electric charge of...

1 C flowing through a conductor each second

54

How is electric potential measured?

in volts

55

How is electric resistance measured?

in ohms

56

Electrons at high ____ have high ____ ____ and high ____ to ___.

voltage; potential energy; capacity; work

57

If electron flow is ____ the circuit resistance is high

inhibited

58

Ohm's law describes..

the manner in which electric currents behave in an electric circuit

59

Formal definition of Ohm's law:

The voltage across the total circuit or any portion of the circuit is equal to the current times the resistance

60

Ohm's law formula:

V=IR (v =volts; I=electric current in amperes; R=resistance in ohms)

61

electric circuits can be reduced to one of 2 basic types:

a series circuit or a parallel circuit

62

a series circuit means...

all circuit elements are connected in a line along the same conductor

63

Rules for series circuits:

1. the total resistance is equal to the sum of the individual resistances
2. the current through each circuit element is the same and is equal to the total circuit current.
3. the sum of the voltages across each circuit element is equal to the total circuit voltage

64

a parallel circuit means...

it contains elements that are connected at their ends rather than lying in a line along a conductor

65

Rules for parallel circuit

1. the sum of the currents through each circuit element is equal to the total circuit current
2. the voltage across each circuit element is the same and is equal to the total circuit voltage
3. the total resistance is the inverse of the sum of the reciprocal of each individual resistance

66

Direct current is...

when electrons are made to flow in one direction along the conductor

67

Alternating current is...

when electrons oscillate back and forth

68

a parallel circuit contains elements that are..

connected at their ends rather than lying in a line along a conductor

69

What is a waveform?

a graph that describes the phenomenon of DC

70

Electric power is measured in...

watts

71

How many watts does a x-ray system usually require?

20-150 kW

72

One watt is equal to ...

1 A of current flowing through an electric potential of 1 V

73

What is the formula for watts?

W = VA

74

Any charged particle in motion creates...

a magnetic field

75

The magnetic field of a charged particle such as an electron in motion is ______ to the motion of that particle.

perpendicular

76

What is electron spin?

A property of rotation caused by electrons that rotate on an axis that is clockwise or counterclockwise

77

What does the electron spin create?

a magnetic field

78

Spinning electric charges also induce ...

a magnetic field

79

What forms the basis of the MRI?

When a proton in a hydrogen nucleus spins on its axis and creates a nuclear magnetic dipole called a magnetic moment.

80

The lines of a magnetic field are always...

closed loops

81

What is a bipolar or dipolar field/

When the lines of a magnetic field do not start or end as the lines of an electric field do. And they always have a north and south pole

82

What creates a magnetic domain?

An accumulation of many atomic magnets with their dipoles aligned

83

What happens if all the magnetic domains in an object are aligned?

It makes a magnet

84

What is magnetic permeability?

the ability of a material to attract the lines of magnetic field intensity

85

What are the 3 types of magnets?

1. naturally occurring
2. artificially induced permanent
3. electromagnets

86

How are magnets classified

according to the origin of the magnetic property

87

What is the best example of a natural magnet?

the Earth

88

Why does the Earth have a magnetic field?

cause it spins on its axis

89

Why do lodestones have a strong magnetism?

because they have remained undisturbed for a long time in the Earth's magnetic field.

90

What are artificial permanent magnets made of?

usually iron

91

What is a good example of an artificial permanent magnet?

a compass

92

How are permanent magnets usually produced?

by aligning their domains in the field of an electromagnet.

93

How can an artificial permanent magnet be destroyed?

By heating it up or hitting it with a hammer

94

What do electromagnets consist of?

wire wrapped around an iron core.

95

How is an electromagnet magnetized?

When an electric current is conducted through the wire, a magnetic field is created. The intensity of the magnetic field is proportional to the electric current. The iron increases the intensity of the magnetic field.

96

How can ALL matter be classified?

according to the manner in which it interacts with an external magnetic field.

97

What are diamagnetic materials?

weakly repelled by either magnetic pole, cannot be artificially magnetized & are not attracted to a magnet. (water and plastic)

98

What are ferromagnetic materials?

strongly attracted by a magnet and can be permanently magnetized by exposure to a magnetic field. (iron, cobalt, nickel) Or (alloy of aluminum, nickel, and cobalt called alnico)

99

What are paramagnetic materials?

very slightly attracted to a magnet & loosly influenced by an external magnetic field. (contrast agents for MRI) -they are somewhere between ferromagnets and nonmagnetics-

100

Define magnetic susceptibility

the degree to which a material can be magnetized

101

The imaginary lines of the magnetic field leave the ___ ____ of the magnet and return to the ___ ____.

north pole; south pole

102

What can be made into magnets by induction?

ferromagnetic objects

103

Any device that converts some form of energy directly into electric energy is called...

a source of potential energy

104

How is electric potential measured?

in units of joules per coulomb, or volts

105

Any charge in motion induces...

a magnetic field

106

a coil of wire is called..

a solenoid

107

Describe an electromagnet

a current carrying coil of wire wrapped around an iron core, which intensifies the induced magnetic field

108

The magnetic field produced by an electromagnet is the same as...

a bar magnet

109

With an electromagnet, can the strength of the magnetic field be adjusted?

Yes. by varying the current through the coil of wire

110

In order to induce a current with the use of a magnet, what needs to be happening?

the magnetic field must be changing

111

What is electromagnetic induction?

an electric current is induced in a circuit if some part of that circuit is in a changing magnetic field.

112

What is considered faraday's law?

electromagnetic induction

113

Do you need physical motion in order to have electromagnetic induction?

No. as long as the current in an electromagnet is increase and decreased, its magnetic field will change and induce a current in the coil.

114

What is a good example of electromagnetic induction?

radio reception

115

Varying magnetic field intensity induces...

an electric current

116

What is the basis of the electric motor?

an electric current that produces a mechanical motion (the motion of a compass needle)

117

What is the basis of the electric generator?

mechanical motion induces electricity in a coil of wire (motion of a magnet near a coil of wire)

118

What type of motor is used with xray tubes?

an induction motor

119

In an xray tube the rotor is made of...

bars of copper and soft iron into one mass

120

in an xray tube the external magnetic field is supplied by...

several fixed electromagnets called stators

121

What powers the rotating anode of an xray tube?

an induction motor

122

In an xray tube no electric current is passed to the rotor. Instead...

current is produced in the rotor windings by induction. The electromagnets surrounding the rotor are energized in sequence, producing a changing magnetic field.

123

The induced current produced in the rotor windings...

generates a magnetic field

124

After a magnetic field is produced in the anode, what causes the rotor to rotate?

the magnetic field attempts to align itself with the magnetic field of the external electromagnets. But, because the electromagnets are being energized in sequence, the rotor begins to rotate, trying to bring the magnetic field into alignment.

125

Other than the anode, what uses the interacting magnetic fields produced by changing electric currents?

the transformer

126

The transformer does not convert one form of energy to another but...

transforms electric potential and current into higher or lower intensity.

127

A transformer...

changes the intensity of alternating voltage and current.

128

A transformer will only operate with...

a changing electric current (AC)

129

A change in voltage of a transformer is directly proportional to...

the ratio of the number of turns of the secondary coil to the number of turns in the primary coil

130

The voltage change across the transformer is proportional to...

the turns ratio

131

A transformer with a turns ratio greater than 1 is a...

step up transformer b/c the voltage is increased or stepped up from the primary side to the secondary side

132

When the turn ratio is less than 1, the transformer is a...

step down transformer

133

If voltage in a transformer is doubled, then the current is...

halved

134

In a step up transformer, the current on the secondary side is ___ than the current on the primary side.

smaller

135

In a step down transformer, the current on the secondary side is ____ than the current on the primary side.

larger

136

What is a closed core transformer?

a transformer built about a square core of ferromagnetic material. The material is not a single piece, but is built up of laminated layers of iron which reduces energy loss making it more effecient.

137

How is an autotransformer constructed?

an iron core with only one winding of wire. the winding acts as both the primary and secondary winding. connections are made at different points on the coil for both primary and secondary sides.

138

Simply put, the autotransformer has...

one winding and varies both voltage and current.

139

The autotransformer is ideal for xray machines. T or F

F. It can only handle a small voltage

140

What is a shell type transformer?

The secondary is wrapped around the primary and has 2 closed cores. Most transformers are shell type.

141

Practical applications of the laws of electromagnetism appear in...

the electric motor, electric generator, and transformer